We have VERY cold water year-round (drilled well), but obviously colder in the winter. Right now (Dec), our water is running either side of 40 F from the well into the pressure tank & the basement, where the mechanicals are located, stays around 45-50F. We heat with wood on the 1st floor. The water at the kitchen sink runs about 46F.

Really, the only place we would want water as cold as it is would be the kitchen sink, otherwise somewhat tempered cold water through out the house would be fine. It would also lighten the load on the water heater.

I have thought about a tank in heated space, etc. Ideas or suggestions other than heating the basement?

I have thought about that, as we run our shower valves at around 90% hot, I'm guessing. Re-piping to that configuration wouldn't be a problem as things are pretty consolidated in the basement. I should add that we have a tankless heater.

Ideally, I would like to bring all the cold (post-kitchen sink) to room temp & have wondered mostly about some sort of retention-type tank.

Regardless of how you temper it it may be costing you anyway you do it because you are taking a created heat source and cooling it in order to raise the temp of the water...if you are using the heat from the wood then there may be no cost but your time involved unless you are buying it...using a bare steel tank, like hj said, the degree of tempering will be an equation based on the amount of time exposed to the ambient air, the size of the tank, the amount of water you use, and the waters temperature...but you better like red water...

The rust issue did cross my mind as a possible issue with a steel tank.

But from my looking around it seems there are also coated as well as non-metallic pressure tanks out there but they are not inexpensive, so it might not be worth the trouble. Cheaper to turn up the water heater a little & maybe add a mixing valve for the showers if need be.

As an academic point though, are there any pressure losses, etc. in having such a tank in the system?